How To Fix: Cannot Access security.debian.org

For a while, my Linux server could not access security.debian.org when updating the system. Security.debian.org houses important security patches, meaning that this is a serious problem. Initially, I thought that the server was just down temporarily. However, it seemed to be down every time that I updated the system. Confusingly, the web-browser had no trouble accessing the security update server, and the server responded to pings.

It turns out that disabling IPv6 solves this issue. Why exactly I do not know, particularly as my network and ISP do not support IPv6 at this time. However, this solution works, and is achieved as follows:

Open /etc/modproble.d/aliases in a text editor (as root user):

sudo gedit /etc/modprobe.d/aliases

Add a # symbol to the start of the following line:

alias net-pf-10 ipv6

The line above should now read:

#alias net-pf-10 ipv6

Save and close the file.

With this done, apt-get (Debian's software update tool) should once again be able to access security.debian.org, unless the server really is down. This can be tested simply by entering the following in a terminal window:

sudo apt-get update

If everything is working, then apt-get will list all of the servers that it has checked for updates, and will not return any errors.